Chop the carrot, celery, and second onion into small pieces. Set aside.

Season both sides of the beef slices with salt and pepper. Spread 1 tablespoon of mustard onto the top side of a beef slice. Top this with 2 slices of bacon,
some pickle pieces, and some onion rings. Top this with one of the ground meat portions. Roll up the beef, making sure the filling gets tucked in as you roll. Tie the rouladen together with cooking-safe string or use toothpicks (or other kind of meat sticks) to hold the rouladen together. Repeat these steps for the remaining 3 slices of beef.

Preheat oven to 325°F.

Heat oil in a roasting pot. Briefly sear the rouladen so that the rouladen develops a nice brown outer crust. Remove rouladen from the pot and set aside.

Add the carrots, celery, and second onion to the roasting pot. Add tomato paste and seasonings. Combine and cook for several minutes. Add wine and bring to a boil. Add broth and bring again to a boil. Add rouladen back to the pot. The rouladen should be covered with liquid. Bring to a boil again, then transfer pot (lid on) to the oven. After 1 hour, turn rouladen over and open the lid of the pot slightly to allow some steam to escape. Cook rouladen this way in the oven for another hour.

Remove rouladen from the pot. Run the sauce through a colander to remove vegetables and bay leaf. Optionally add a little thickening agent to the sauce to thicken the sauce according to your taste. Season sauce with salt and pepper to taste. Remove string/toothpicks from the rouladen before serving.